I had this issue with my Acer Iconia A500. I couldn’t see the external SD card with the default install of ES File Explorer. Simple fix for this issue. Go into the settings of ES File Explorer and change the root directory to just ‘/’ instead of ‘/sdcard/’ (exclude the quotation marks) which shows the internal memory only. This will allow you to navigate to the root of the storage directory. If you are using an Acer Iconia A500 you can set the root directory to ‘/mnt/’ which is the directory where you can access the sd card, the internal memory and USB memory if you have it plugged in. Note that the internal storage is named ‘sdcard’ and the sd card is called ‘sdcard-ext’. Hope this helps some people.